This invention is in the general field of electrochemical conversion using cells, particularly high energy density batteries.
Many devices that require electricity from a battery are limited in usefulness by the battery""s lifetime. Both weight and size (in other words, energy density) can be limiting factors on battery life, particularly for small devices. In particular hearing aids and many other devices would be enhanced by increasing the battery""s energy density. For example, many devices could be further miniaturized if a smaller battery that gave reasonable energy density were available.
Accordingly, there is a general need to generate as much electrical energy as possible from a battery having a limited volume.
I have discovered that the use of certain borides can produce a high energy density cell. High power densities are also achievable, for example, by using borides that are reasonably good conductors of electricity. I have further discovered that the performance of borides such as titanium boride is signigicantly improved by adding a halide, such as a fluoride, in the electrolyte system. Without wishing to bind myself to a specific theory, I conclude that even though the borides or the resulting borates in question may have desirable properties,xe2x80x94e.g., high energy density and high conductivity and aqueous compatibility, if the resutling borate (e.g., titanium borate) is very insoluble, it can coat the boride and thereby degrade battery performance. In particular, such coating reduces the available power and it causes the. battery to fail prematurely, before substantially all of the material is oxidized. Batteries having a halide-(particularly fluoride-) containing electrolyte will perform substantially better. Again, without binding myself to a particular theory, if fluoride ion is present in this system, the highly soluble complex anions of titanium hexafluoride and boron tetrafluoride are formed. These soluble ions now diffuse away from the boride particle and allow further reaction until the boride is more effectively
Accordingly one aspect of the invention generally features a battery comprising an anode and a cathode in electrical communication; the anodic electrochemical storage medium comprises as a reduced species: a) boron; b) at least one reduced boron-containing compound; or c) both. The reduced species is oxidizable to an oxidized boron-containing compound in a reaction which yields an electric current, and the oxidized boron-containing compound is soluble in the electrochemical storage medium as the battery is discharged.
The battery is particularly adapted to the use of aqueous systems for the storage medium. Particularly preferred reduced boron-containing compounds are borides. As noted, it is particularly useful to include a halide (e.g.. a fluoride such as may be provided by sodium fluoride) in the the anodic medium. Alternatively, or in combination with the fluoride, the halide may be chloride (e.g., sodium or potassium chloride).
Preferred borides are conductive to enhance the overall conductivity and therefore the deliverable current. Transition metal borides are particularly preferred. Titanium diboride and vanadium diboride are preferred.
Preferred cathodes comprise a structure that is exposed to oxygen (e.g. to air), such as those in which cathode is also exposed to an aqueous electrolyte, and oxygen is reduced to xe2x80x94OHxe2x88x92.
The anodic medium may further comprise a borohydride in addition to boron or the reduced boron-containing compound. It may also comprise a metallic boride such as FeB or NiB2. The anodic storage medium may also further comprise a conductivity enhancer such as graphite. The enhancer may itself be oxidized to provide addition current during oxidation of the reduced boron-containing compound, or it may be inert. The anodic medium may comprises a combination of borides. For example, mixtures of the borides are contemplated to achieve desired combinations of energy density and conductivity, depending on the application. For example, a low conductivity boride may be mixed with a higher conductivity boride to achieve a desired energy density and conductivity.
The oxidized boron-containing compound may be a boron halide or a boron oxyhalide, a borate or polyborate. Preferably, the the oxidized boron-containing compound is conductive.
Other borides that may serve as the reduced boron-containing compound include aluminum borides. See also, table 1.
The anodice stoarage medium may further comprise EDTA, in addition to the boron or reduced boron-containing compound. The oxidized boron-containing compound may include a metal oxide and a borate. The oxidized boron-containing compound may contains a combination of corresponding metal oxides, halides and oxyhalides.
Typically the storage medium is an aqueous, but the invention may also be used in non-aqueous systems. Another way to enhance conductivity and thereby increase current is to use a conductive electrolyte. Conductivity enhancers, such as borohydrides or metallic borides, may also be added to the medium to both enhance conductivity and to contribute, to some extent, to electrical output. Alternatively, inert conductivity enhancers, such as graphite or other conductive carbon formulations may be used.
The electrochemical reaction is improved by alkaline pH, so the storage medium preferably has a pH above 8.5, and most preferably it has a pH above 11.0. Typically, an alkali metal hydroxide is added to the storage medium to provide conductivity as well as to control pH.
As an alternative to the so-called air or breathing cathode, the cathode may include an oxygen-containing oxidizing compound such as compounds is selected from ferrates, MnO2, CrO3, KMnO4, LiCoO2, NiOOH, peroxides, perhalates, perchlorate, chlorates, bromates, perbromates, iodates, periodates, hypochlorites and chlorites.
Alternatively, the cathode may comprise a non-oxygen containing oxidizing compound, such as a high valency metal or an interhalogen or a metal-halide in which the metal can be reduced to a lower valence.
The invention also features methods of generating a current by contacting a load to any of the above described batteries.
Without binding myself to a specific mechanism of action or limiting myself to one specific advantage of the invention, high (mass) density is important to achieve high energy density. Another important factor is lower weight per available mole of electrons. The borides generally provide a favorable balance of these factors compared to a number of other materials, such as lithium or zinc.
Moreover, individual borides have other important characteristics. Titanium diboride is safe and environmentally acceptablexe2x80x94the final products of titanium boride discharge in a basic medium are essentially borax and titanium dioxide, both of which have a relatively low environmental impact. Even the staring materials, the borides themselves, are somewhat refractory and also relatively benign environmentally. Other borides, such as vanadium boride with a high density of 5.1 g/cc, may be used.